A spin liquid is a novel quantum state of matter with no conventional order parameter where a finite charge gap exists even though the band theory would predict metallic behavior. Finding a stable spin liquid in two or higher spatial dimensions is one of the most challenging and debated issues in condensed matter physics. Very recently, it has been reported that a model of graphene, i.e., the Hubbard model on the honeycomb lattice, can show a spin liquid ground state in a wide region of the phase diagram, between a semi-metal (SM) and an antiferromagnetic insulator (AFMI). Here, by performing numerically exact quantum Monte Carlo simulations, we extend the previous study to much larger clusters (containing up to 2592 sites), and find, if any, a very weak evidence of this spin liquid region. Instead, our calculations strongly indicate a direct and continuous quantum phase transition between SM and AFMI.

The metal-insulator transition has been a subject of intense research since Nevil Mott has first proposed that the metallic behavior of interacting electrons could turn to the insulating one as electron correlations increase. Here, we consider electrons with massless Dirac-like dispersion in two spatial dimensions, described by the Hubbard models on two geometrically different lattices, and perform numerically exact calculations on unprecedentedly large systems that, combined with a careful finite size scaling analysis, allow us to explore the quantum critical behavior in the vicinity of the interaction-driven metal-insulator transition. We find thereby that the transition is continuous and determine the quantum criticality for the corresponding universality class, which is described in the continuous limit by the Gross-Neveu model, a model extensively studied in quantum field theory. We furthermore discuss a fluctuation-driven scenario for the metal-insulator transition in the interacting Dirac electrons: the metal-insulator transition is triggered only by the vanishing of the quasiparticle weight but not the Dirac Fermi velocity, which instead remains finite near the transition. This important feature cannot be captured by a simple mean-field or Gutzwiller-type approximate picture, but is rather consistent with the low energy behavior of the Gross-Neveu model.

The minute surface structure and microvessels observed by magnifying endoscopy were related to histopathological findings. Magnifying endoscopy is valuable for predicting the histological nature in the diagnosis of early gastric cancer.

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The magnetic-field dependence of optical spectra and their anisotropy have been investigated for a single crystal of Pr 1Ϫx Ca x MnO 3 ͑xϭ0.4͒ at 30 K. The charge-ordered ͑CO͒ state is transformed into a ferromagnetic metallic state by a magnetic field of 6.5 T, which is manifested in a huge change of optical spectra over a wide photon-energy region ͑0.05 eV-3 eV͒. The observed change in magnitude and anisotropy of the optical spectra with the external magnetic field has been elucidated in terms of the spin-and orbital-ordering structures in the CO state. ͓S0163-1829͑98͒51316-1͔